The present invention involves an improved chopper for chopping continuous or very long loose items such as fiber, fiber strands, yarn, wire, string, ribbon, tape and the like by pulling the item(s) into the chopper while the loose items are held tightly against the surface of a rotating backup roll and carrying the item(s) on into a nip between a rotating blade roll and the rotating backup roll where they are separated into short pieces. More specifically the present invention involves an improved chopper having an improved blade roll and a method of using the improved chopper to make chopped products.
It has long been known to chop continuous fibers or fiber strands into lengths of about 1-5 inches or shorter. Billions of pounds of such product including chopped glass fibers and fiber strands are produced each year in process and chopping apparatus such as disclosed in U.S. Pat. Nos. 5,970,837, 4,551,160, 4,398,934, 3,508,461, and 3,869,268, the disclosures of which are incorporated herein by reference. The choppers disclosed in these patents comprise a blade roll containing a plurality of spaced apart blades for separating the fibers into short lengths, a backup roll, often or preferably driven, which the blades work against to effect the separation and which pulls the fibers or fiber strands and in some cases, an idler roll to hold the fibers or fiber strands down onto the surface of the backup roll. In the chopped fiber processes disclosed in these patents, the chopper is often the item most limiting the productivity of the processes. These processes typically operate continuously every day of the year, 24 hours each day, except for furnace rebuilds every 5-10 years.
Many of the above choppers use a blade roll made using an elastomeric material layer such a rubber, polyurethane, or other material having similar elastomeric properties, for holding spaced apart blades in spaced apart slots in the elastomeric layer, see U.S. Pat. Nos. 4,083,279 and 4,287,799. In a large operation, many blade rolls must be inventoried to service a plurality of choppers making several different products at any one time, one of the differences in the chopped products being length of the chopped product. To minimize the number of blade rolls that must be inventoried to support a substantial operation, it has been the practice to form blade slots on a 0.25 inch (6 mm) or 0.5 inch (12-13 mm) center to or 0.5 inch (12-13 mm) center to center spacing in all of the backup rolls. The final blade roll are then made up to make a particular product or group of products close to the time they are needed. In making up the blade rolls, blades are placed only in the slots appropriate for making the chopped length desired for chopper and product to be produced with those blade rolls. When making a 1.25 inch (31 mm) long product, several slots are left empty between the blades that are inserted on 1.25 inch blade edge to blade edge spacings. Dummy filler pieces were normally placed in the empty slots to present a smooth surface to the backup roll and to maintain pressure on the sides of the blades setting in the slots of the blade roll. These dummy filler pieces filled the slots the same as blades, but ended at or just below the top surface of the elastomeric layer of the blade roll.
These choppers run at speeds such that the surface speed of the backup roll and the edge of the blades move at thousands of feet per minute, i.e. from 2,000 to more than 6,000 feet per minute, such as 7,000 to 10,000 feet per minute. It has not been discovered that this practice of using dummy filler pieces in the slots between blades in the blade roll, while necessary to maintain the integrity of the blades and elastomeric layer of the blade roll, nevertheless causes vibration at these chopping speeds and when the distance between the blades is about one inch or longer. More than half of the fiber produced in the United States is one inch long or longer and a substantial, and growing, amount of fiber produced in elsewhere in the world is about one inch long or longer. This vibration, chatter, reduces the life of the backup roll substantially over what it could be without vibration, is very noisy, reduces the quality of the chopped fiber and causes significant scrap because of sling-off of good fiber onto the forming room floor and onto the chopper itself and probably leads to equipment damage such as bearings and shafts supporting the blade roll and the backup roll and other parts. Because of the cost of shutting a chopper down to replace any part on the chopper—all bushings feeding the chopper make only scrap while the chopper is down and operating equilibrium is upset causing a lowered operating efficiency for a period of time after startup—it is cost effective to also change the blade roll while the chopper is down even though it may have many hours of life left. A solution to this problem has been sorely needed for a long time.